1. Field of the Invention
The present invention refers to roofing granules and roofing products having biocidal activity.
2. Brief Description of the Prior Art
Asphalt shingles are conventionally used in the United States and Canada as roofing and siding materials.
Asphalt shingles can be classified into two types of shingles according to the nature of the reinforcement. “Organic” shingles contain cellulose or wood fiber as a thick fiber felt. “Glass fiber” shingles contain a nonwoven mat of glass fibers held together by a binder that is insoluble in water.
In the manufacture of organic shingles, a continuous web of organic fiber felt is fed from a supply roll to an accumulating device made up of several rollers, and then immersed in a first liquid asphalt bath having a temperature of about 250° C. After leaving the first liquid asphalt bath, the felt passes through a second accumulating device so that the felt can absorb excess asphalt and cool slightly. The so-impregnated felt is then coated with molten asphalt on each of its two faces, which ultimately become respectively the upper and lower faces of the web. Roofing granules are distributed on the upper face, and an anti-adhesive agent, for example, talc, is applied to the lower face. The resulting web passes between the rollers of a cold calendar so as to partially embed the roofing granules in the hot asphalt layer on the upper face of the web, and the subsequently cooled product is collected in the form of rolls or of sheets cut to the desired dimensions.
Except for the first stage of impregnation, which is omitted, the manufacture of the glass fiber shingles is carried out in the same way.
In the shingle, the asphalt functions principally to make material impervious to water. It is also used to support the granules and to give strength to the material. The highly ductile character of the asphalt-impregnated felt makes it possible to obtain a flexible product. In general, the longevity of the shingle increases with the quantity of asphalt employed.
The roofing granules, in general formed from mineral materials, serve to provide the shingle with durability. They protect the asphalt from the effects of solar radiation (in particular from the degradative effects of ultraviolet rays) and of the environment (wind, precipitation, pollution, and the like), and contribute to better reflection of incident radiation. The granules, moreover, are typically colored, naturally or artificially by way of the application of pigments, to meet the aesthetic requirements of the user.
However, it is not unusual to see unattractive green, brown or black spots appearing on the surface of asphalt shingles of buildings located in temperate climates. These spots are due to microorganisms, mainly algae of the Gloeocapsa genus, which benefit from conditions favorable to their growth found in temperate climates. These conditions include heat, moisture and nutrients. The essential biogenic salts may be provided by the mineral granules themselves, but also may be supplied by organic matter which settles on the shingles. The unattractiveness of these spots, all the more noticeable when the color of the shingle is a light one, is not the only disadvantage. In addition, the resulting darkening of the surface causes an increase in the absorption of the solar radiation, which in turn reduces the effectiveness of the shingles as thermal insulation, and decreases its service life.
To address this problem, algae-contaminated shingles can be treated with suitable biocides. However, the complete elimination of the algae is difficult, and requires the treatment of the entire building, including seemingly healthy surfaces. Even by using a powerful biocide such as sodium hypochlorite, the prophylactic effect is not permanent, because the roof is subsequently scrubbed by weather-borne water. Moreover, certain green algae particularly resistant to biocides can re-colonize previously treated surfaces, thus requiring additional treatments, at regular intervals, to limit their reappearance.
Other methods known to prevent the appearance of the undesirable algae growth are based on the incorporation of algaecide in the shingle. For example, it has been suggested that granules include metal compounds in the form of zinc oxide or sulfide (U.S. Pat. No. 3,507,676), or copper oxide (U.S. Pat. No. 5,356,664), or that a mixture of copper oxide and zinc oxide (U.S. Patent Publication 2002/0258835 and U.S. Patent Publication 2002/0255548) be incorporated in the asphalt.
It has also been suggested to disperse a granular or pulverulent material containing an algaecide on the surface of the shingle (JP-A-2004162482).
U.S. Pat. No. 6,245,381 suggests adding a biocide in the form of salt or of chelate starting from Cu2+, Zn2+ and Sn2+ ions complexed with an organic binder anion in asphalt during the manufacture of the shingle.
Another approach has been to employ photocatalytic particles as biocidal agents. The photocatalytic effect has been employed to provide self-cleaning glass and other ceramic materials. For example, U.S. Pat. No. 6,037,289 discloses a substrate provided with photocatalytic anatase titanium dioxide that is at least partially crystalline, and has a mean size of between 5 and 80 nm. The coating can include an inorganic binder, such as an amorphous or partially crystalline oxide, or a mixture of oxides, such as oxides of silicon, titanium, tin, zirconium or aluminum, which can serve as a matrix for the photocatalytic titanium oxide. Alternatively, a partly organic binder can be used, such as a binder based on epoxide-containing alkoxysilanes. Similarly, U.S. Pat. No. 6,465,088 discloses a substrate such as a glass or acrylate glazing material covered with a photocatalytic coating including crystallized particles having photocatalytic properties and a mineral binder comprising at least one oxide of a metal having photocatalytic properties. U.S. Pat. Nos. 6,569,520 and 6,881,702 disclose a photocatalytic composition and method for preventing algae growth on building materials such as roofing granules. A plurality of photocatalytic particles, such as anatase titanium dioxide, is dispersed in a silicate binder to form an exterior coating for a substrate such as a roofing granule or concrete surface. At least a portion of some of the photocatalytic particles is exposed on the surface of the coating.
There is a continuing need to prevent the appearance of undesirable algae growth on roofing shingles and other roofing materials.